Time-resolved changes in intracellular calcium following depolarization of rat brain synaptosomes

1. Changes of cytoplasmic free calcium levels ([Ca2+]i) in isolated rat brain nerve terminals (synaptosomes), previously loaded with the fluorescent intracellular calcium indicator Fura-2, were measured 1-2 ms after depolarization with elevated K+ by stopped-flow fluorescence spectroscopy. 2. In physiological saline (PSS) containing 4 mM-K+, intraterminal Ca2+ was estimated to be in the range 150-250 nM. Depolarization of the nerve terminals with elevated external K+ in the presence of Ca2+ induced a prompt rise in [Ca2+]i, which occurred in two phases. No change in [Ca2+]i was seen when the terminals were depolarized in nominally Ca(2+)-free solutions, and only a small change was seen when the terminals were acutely exposed to Ca2+ in 4 mM-K+. 3. Predepolarization of the nerve terminals with K+ in nominally Ca(2+)-free solutions several seconds before the introduction of Ca2+ greatly decreased the magnitude of the fast phase, whilst leaving the slow phase largely intact. 4. In Na(+)-depleted nerve terminals, the fast phase of K(+)-stimulated Ca2+ uptake was essentially unaltered, but the slow phase of Ca2+ uptake was dramatically reduced. 5. The rapid phase of K(+)-stimulated uptake displayed voltage-dependent inactivation (tau approximately 50 ms at -10 mV), and the rate of inactivation was accelerated with increasing depolarization. In contrast, at constant [K+]o, increasing [Ca2+]o had little or no effect on the rate of inactivation, but did increase the initial rate of Ca2+ uptake. 6. The dihydropyridine calcium channel blockers nifedipine and nitrendipine had little effect on either component of Ca2+ uptake. However, the inorganic Ca2+ channel blockers La3+, Cd2+, and Co2+ were potent blockers of the fast phase of Ca2+ uptake, but blocked the slow phase only at higher concentrations. No consistent effect of the peptide neurotoxin omega-conotoxin was observed on either component of the Ca2+ rise. 7. These studies demonstrate that the dynamics of depolarization-activated intraterminal Ca2+ changes can be studied on a millisecond time scale in isolated nerve terminals. Moreover, our results indicate that two pathways contribute to depolarization-induced [Ca2+]i changes, namely a voltage-activated, inactivating Ca2+ channel, possibly of the N-type, and Na(+)-Ca2+ exchange operating in the 'reverse' mode.